It has been known in the past that by selection of varied geometrical cross sectional configurations, formed catalytic particles may be designed to offer shorter diffusion paths for reactants and/or products which may be desirable in, for example, a diffusion controlled reaction. Additionally, catalytic particle configuration has been employed in the past to reduce the pressure drop encountered across a packed catalyst bed, and/or to reduce catalyst loss due to breakage, abrasion or crushing during handling, sizing or when severe process conditions are encountered. As a specific example, tubular extrudates have been employed in the past to improve the surface area-diffusion path presented by such catalytic particles, as well as to reduce the pressure drop encountered when reactants are passed through a catalytic bed. However, such tubular, cylindrical extrudates have the disadvantage of low crushing strength whereby during handling or use fracturing of the delicate tubular cylindrical structures results in fragmentation of the catalyst particle which will result in pressure drop increases in certain instances. The novel shapes of the present invention exhibit greater fracture resistance and, additionally, have a reduced tendency toward end blockage by an adjacent particle in a randomly packed catalyst bed when compared to prior art tubular cylinder structures.
In accordance with one aspect of the present invention, novel tubular extrudate cross sectional configurations are provided which are specifically designed to reduce catalyst loss due to breakage, abrasion, or crushing forces which may be encountered in handling, sizing or under severe process conditions. It has been shown in the past that by intentionally altering the shapes of formed particles from that most often utilized in fixed bed applications i.e. a solid cylinder, the pressure drop or resistance to flow encountered across said bed can be reduced. In the case of formed catalytic particles, configurational changes so alter the surface to volume ratio that the resultant design offers shorter diffusion paths for reactants and/or products which may be desirable in, for example, a diffusion controlled catalytic reaction. Alternatively, changes in particle forms have been employed in the past to reduce loss due to breakage, abrasion or crushing during handling, sizing or when severe process conditions are encountered. As a specific example, the prior art discloses that tubular extrudates, i.e. formed particles possessing both cylindrical external perimeters and cylindrical internal cores or holes reduce the pressure drop across a packed bed of the same height relative to solid cylinders by virtue of the increase in bed void volume. Additionally in catalytic applications these tubular shaped entities have been employed to improve heat and mass trasfer rates by virtue of their surface to volume enhancement, where such factors are controlling. However, such tubular, cylindrical extrudates have the disadvantage of low crushing strength relative to their solid counterparts. Consequentially, during handling or use fracturing of the delicate tubular cylindrical structures results in fragmentation of the particles which, in commercial applications, ultimately results in increased pressure drop as a result of an increase in resultant fines or dust. Conversely, the neoteric shapes of the present invention offer improvements in the resistance to fluid flow or pressure drop across a packed bed, and also improvements in heat and mass trasfer properties as a consequence of their favorable surface to volume ratios, which improvements are of special significance in catalytic applications. In addition, and entirely unexpected, the unique shapes herein below described, simultaneously exhibit a resistance to loss due to breakage, abrasion or crushing forces which may be encountered in handling, sizing or under severe process conditions. Consequently, the extrudate shapes in accordance with the present invention offer, by design, improvement over solid, i.e. non-tubular, cross sections with respect to reduced pressure drop across a fixed bed, and at least one or more of these shapes also evidences a reduced tendency toward end blockage by an adjacent particle in a ramdonly packed bed.
For purposes of the present invention "tubular" is intended to include configurations which in addition to hollow cylinders include hollow or apertured structures having non-circular external perimeters, e.g. rectangular, triangular or other geometrical configurations such as multiple bulbous protrusions connected by concave surfaces or the like, specific examples of which are shown in the accompanying Figures and Drawings.